1. Field of the Invention
The present invention relates to a beam shaping lens, a lens part relating to it, a mounting plate, an optical head, an optical information recording and reproducing apparatus of irradiating light on an optical disk from the optical head to record or reproduce information, and a computer, an image recording and reproducing apparatus, an image reproducing apparatus, a server and a car navigation system using this.
2. Related Art of the Invention
An optical disk called a DVD (Digital Versatile Disk) as a high-density and large-capacity optical information recording medium is available in the market. Recently, such an optical disk is rapidly becoming widespread as a recording medium of recording images, music and computer data. In recent years, researches on a next-generation optical disk of further enhanced recording density are underway in various places. The next-generation optical disk is anticipated as a recording medium replacing a VTR (Video Tape Recorder) video tape which is currently mainstream, and is under development at a rapid rate.
An optical head of recording or reproducing information on the optical disk comprises a light source, an objective lens of condensing a beam emitted from the light source on the optical disk and a detector of detecting the beam reflected from the optical disk. A semiconductor laser as the light source has the beam radiated from an end face of a thin active layer, and so the beam is in an elliptic form and a ratio between its minor axis and major axis is approximately 1:3 or so. When recording the information on the optical disk, it is desired, from a viewpoint of improving usability of the light, to shape the elliptic beam into a circular form.
Next, first to fourth conventional examples of shaping the beam will be described.
FIG. 23 is a first conventional example of shaping the beam into a circular form (refer to Japanese Utility Model Laid-Open No. 63-118714 (FIGS. 1 and 4) for instance), which shows a schematic diagram of an optical head 309 using a beam shaping lens 302. An elliptic diverging beam emitted from a light source 301 is rendered as a circular diverging beam by the beam shaping lens 302 described later, and is transmitted through a branching prism 303, rendered as a parallel beam by a collimator lens 304, reflected by a mirror 305, condensed by an objective lens 306 and irradiated on an optical disk 310. The beam reflected on the optical disk 310 traces an inverse route to be reflected on the branching prism 303 and detected by a detector 308.
Both sides of the beam shaping lens 302 are cylindrical surfaces, where the beam is refracted and expanded by the cylindrical surface along a minor axis direction of the beam, and is transmitted along a major axis direction without changing its broadening angle so as to shape the elliptic beam into a circular beam.
FIG. 24 is a second conventional example using cylindrical lenses 302a and 302b separately provided spatially (refer to Japanese Utility Model Laid-Open No. 63-118714 (FIGS. 1 and 4) for instance). It is possible, even in such a configuration, to shape the elliptic beam into the circular beam as with the first conventional example.
FIG. 25 is a third conventional example of shaping the beam into a circular form with the lens (refer to Japanese Patent Laid-Open No. 2002-208159 (FIG. 1) for instance). As for a beam shaping lens 402, its first plane 402i and second plane 402o are cylindrical surfaces, where the beam is refracted and expanded by the cylindrical surface along the minor axis direction of the beam, and is transmitted along the major axis direction without changing its broadening angle so as to shape the beam. As the first plane 402i is an aplanatic surface, no aberration arises. A distance on an optical axis from a luminous point of a light source 401 to the first plane 402i is equal to thickness of the beam shaping lens 402 on the optical axis. Therefore, the beam in the minor axis direction vertically gets incident on the second plane 402o so that no aberration arises. The second plane 402o has a cross section of a plane vertical to a central axis of the cylindrical surface (the plane parallel to space of FIG. 25) which is non-circular arc (hereafter, such a cylindrical surface is referred to as a cylindrical surface of a non-circular cylindrical plane). As the second plane 402o is the cylindrical surface of a non-circular cylindrical plane, an aberration of about the same degree as in the major axis direction is generated in the minor axis direction so as to have a spherical aberration of axial rotation symmetry. The spherical aberration generated on the beam shaping lens 402 is eliminated by a collimator lens 404.
FIG. 26 is a fourth conventional example of shaping the beam into a circular form with a prism, showing a schematic diagram of an optical head 509 using a beam shaping prism 502. The diverging beam emitted from a light source 501 is rendered as a parallel beam by a collimator lens 504, is refracted by the beam shaping prism 502 along the minor axis direction of the beam so as to shape the elliptic beam into the circular beam. The circular beam is transmitted through a branching prism 503, reflected by a mirror 505, condensed by an objective lens 506 and irradiated on an optical disk 510. The beam reflected on the optical disk 510 traces an inverse route to be reflected on the branching prism 503, transmitted through a detection lens 507 and detected by a detector 508.
However, the beam shaping lens 302 of a first conventional example shown in FIG. 23 has its beam shaping magnification limited to 1.2 times or so. As for the cylindrical surface of the beam shaping lens 302, the cross section of the plane vertical to the central axis of the cylindrical surface (the plane parallel to space of FIG. 23) is substantially a circular arc (hereafter, such a cylindrical surface is referred to as a cylindrical surface of a circular cylindrical plane). If the beam shaping magnification is set at twice or larger to obtain a substantially circular beam, a high-order aberration of 0.06λ or more (λ is a wavelength of the beam) arises because it is the cylindrical surface of a circular cylindrical plane, and so it not practical.
In the case of using the cylindrical lenses 302a and 302b of a second conventional example shown in FIG. 24, the high-order aberration arises likewise if the beam shaping magnification is set at twice or larger. As the cylindrical lenses 302a and 302b are spatially separate, there is also a problem that the spacing between them varies according to temperature change so that the beam shaping magnification varies and the aberration deteriorates.
The present invention has been made in view of the problems, and an object thereof is to provide the beam shaping lens of which shaping magnification is enhanced and high-order aberration is held down, and the lens part, optical head and so on using it. Another object is to provide the beam shaping lens of which shaping magnification and aberration are stable, and the lens part, optical head and so on using it.